diff --git a/CMakeLists.txt b/CMakeLists.txt
index 0f46036..bc67965 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -57,8 +57,6 @@ option(MSVC_STHREADED_RT "MSVC: Build with single-threaded static runtime libs (
 ###
 if(YAML_CPP_BUILD_OLD_API)
 	set(API_DEFINE "#define YAML_CPP_OLD_API")
-else()
-	set(API_DEFINE "#undef YAML_CPP_OLD_API")
 endif()
 
 configure_file(${YAML_CPP_SOURCE_DIR}/include/yaml-cpp/yaml.h.cmake ${YAML_CPP_SOURCE_DIR}/include/yaml-cpp/yaml.h @ONLY)
diff --git a/include/yaml-cpp/yaml.h.cmake b/include/yaml-cpp/yaml.h.cmake
index 9f2c360..1c93755 100644
--- a/include/yaml-cpp/yaml.h.cmake
+++ b/include/yaml-cpp/yaml.h.cmake
@@ -5,6 +5,7 @@
 #pragma once
 #endif
 
+#undef YAML_CPP_OLD_API
 @API_DEFINE@
 
 #include "yaml-cpp/parser.h"
diff --git a/src/null.cpp b/src/null.cpp
index 240de33..61a634b 100644
--- a/src/null.cpp
+++ b/src/null.cpp
@@ -1,6 +1,6 @@
 #include "yaml-cpp/null.h"
 
-#if YAML_CPP_OLD_API
+#ifdef YAML_CPP_OLD_API
 #include "yaml-cpp/old-api/node.h"
 #endif
 
@@ -8,7 +8,7 @@ namespace YAML
 {
 	_Null Null;
 
-#if YAML_CPP_OLD_API
+#ifdef YAML_CPP_OLD_API
 	bool IsNull(const Node& node)
 	{
 		return node.Read(Null);
diff --git a/src/parser.cpp b/src/parser.cpp
index c131b14..8f2725e 100644
--- a/src/parser.cpp
+++ b/src/parser.cpp
@@ -9,7 +9,7 @@
 #include <sstream>
 #include <cstdio>
 
-#if YAML_CPP_OLD_API
+#ifdef YAML_CPP_OLD_API
 #include "yaml-cpp/old-api/node.h"
 #include "old-api/nodebuilder.h"
 #endif
@@ -58,7 +58,7 @@ namespace YAML
 		return true;
 	}
 
-#if YAML_CPP_OLD_API
+#ifdef YAML_CPP_OLD_API
 	// GetNextDocument
 	// . Reads the next document in the queue (of tokens).
 	// . Throws a ParserException on error.
diff --git a/util/parse.cpp b/util/parse.cpp
index 0653c2a..1187fd3 100644
--- a/util/parse.cpp
+++ b/util/parse.cpp
@@ -37,7 +37,7 @@ public:
 void parse(std::istream& input)
 {
 	try {
-#if YAML_CPP_OLD_API
+#ifdef YAML_CPP_OLD_API
 		YAML::Parser parser(input);
 		YAML::Node doc;
 		while(parser.GetNextDocument(doc)) {